I was recently traveling in a remote mountain region and hiking alongside a very small river. What caught my attention was the fish in the small ponds under the waterfalls.
These ponds were just a couple of square meters in size and each one housed between 5 and 10 fish.
The course (do you mean flow?) of the river combined with the waterfalls mean it was basically out of question that they could swim / jump upstream.
In such small mountainous rivers, how do fish remain in the upper parts over the course of hundreds of years. I mean the size of the ponds and the amount of fish did not look to me like it could be sustainable for a population. And there should be the occasional one going down.
Are there examples or situations where the upstream population becomes extinct?